pgsql: Update version 11 release notes.

Update version 11 release notes.

Remove description of commit 1944cdc98, which has now been back-patched
so it's not relevant to v11 any longer. Add descriptions of other
recent commits that seemed worth mentioning.

I marked the update as stopping at 2018-07-30, because it's unclear
whether d06eebce5 will be allowed to stay in v11, and I didn't feel like
putting effort into writing a description of it yet. If it does stay,
I think it will deserve mention in the Source Code section.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7a46068f47a2e407d80d9d552727dc102188bec2

Modified Files
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doc/src/sgml/release-11.sgml | 113 +++++++++++++++++++++++++++++++++----------
1 file changed, 88 insertions(+), 25 deletions(-)